Evidence for In-Plane Tetragonal c-axis in Mn3Ga Thin Films using Transmission Electron Microscopy

Tetragonal Mn3Ga films show exceptional magnetic and electronic properties, i.e., a unique combination of low magnetization, high uniaxial anisotropy, high Curie temperature, high spin polarization and low Gilbert damping, which make them very promising as ferromagnetic electrodes in Spin-Transfer-Torque Magnetic RAMs [1]. A few recent works have demonstrated epitaxial growth of this phase on MgO, SrTiO3 or with Cr or Pt underlayers [2,3]. We have grown MnxGa1-x (x=0.75,0.70) films with thickness 10 - 40 nm by RF sputtering on SrTiO3(100) and fully characterized the films by structural and magnetometric techniques (X-Ray Diffraction - XRD; High Resolution Transmission Electron Microscopy - HRTEM; Atomic Force Microscopy - AFM; SQUID Magnetometry). For both compositions and in all the considered thickness range the thin films exhibit perpendicular magnetic anisotropy with coercive fields between 1 - 2 T. HRTEM and XRD reveal that 40 nm samples grown at 300-350 °C show predominately the tetragonal phase epitaxial with perpendicular c-axis, but a fraction of in plane c-axes is also present; this structure results in a secondary magnetic component in the perpendicular magnetization. Growth at 300 °C with a reduced thickness or Mn concentration significantly decreases the presence of c-axis in the film plane, thus improving the magnetic properties. TEM is of critical importance in characterizing these materials, since conventional XRD cannot always identify the presence of additional crystallographic orientations although they can still affect the magnetic properties. Our study points to ways that the microstructure of these thin films can be controlled, which is critical for utilization of this material in spintronic devices. [1] B. Balke et al., Appl. Phys. Lett. 90 (2007) 152504 [2] H. Kurt et al., Phys. Status Solidi B 248 (2011) 2338 [3] M. Li et al., Appl. Phys. Lett. 103, 032410 (2013)